The British screenwriter has sparked speculation he may depart the ITV period drama to focus on his new US TV show 'The Gilded Age', for which he has penned the debut episode, as he may not be able to work on both series at the same time.

He explained to The New York Times newspaper: ''If I'm doing a series at NBC, I would not be able to write all of 'Downton' and all of that series at the same time. I would hope that by the time all the hurdles have been cleared, the timing makes it so I can then concentrate on the new series.

''If 'Downton' goes on - of course, that's not my decision - then it would be with other writers. Perhaps with me supervising, but with other writers... I know I would not be able to write 11 hours of 'Downton' and 10 hours of 'The Gilded Age', or whatever it is, side by side.''

Although both Julian and the show have won numerous awards and 'Downton' has become an international hit with viewers, he believes the programme's ''life'' is coming to an end - despite having been renewed for a fourth season.

He continued: He acknowledged: ''My own belief is that these things have a life, and one of the tricks is to recognise when it's time to come to an end. But we haven't made a decision when that will be. Some things go on for 20 years, don't they, but I just don't see 'Downton' being one of them.''